1. Field of the Disclosure
The disclosure relates generally to flow meter monitoring and control assemblies and more specifically to the optimization of purge gas systems with a wireless rotameter.
2. Description of the Related Art
In oil refineries and in other oil and gas processing plants, relief devices that can emit hydrocarbons are connected to a flare or vent network. The flare or vent network ends up in a flare or vent stack which continuously emits the hydrocarbons to atmosphere. A vent stack does not burn the hydrocarbons and simply disposes them to the atmosphere at a safe location. Hence gas flow emitted by a vent stack is typically small. When the gas flows to be continuously disposed are quite high, the gas needs to be combusted before disposing it to the atmosphere. This consumption is achieved in a flare stack. The hydrocarbons are continuously combusted at the flare stack tip before disposing to atmosphere.
If the flow of gas to vent or flare stack stops for some reason, there is a possibility of air ingress into the flare stack and into the vent knockout drum and other regions of the vent network. This can result in an explosive mixture of air and hydrocarbons in the vent or flare network, which can be catastrophic. If air enters the flare stack, a release of process gas could result in a combustible mixture present within the flare in a restricted zone. The ignition of this air and gas mixture can result in an explosion and cause severe equipment damage. In addition, oxygen concentrations in the stack exceeding approximately 6% could allow the ingress of the flame front.
One of the ways to avoid air ingress into the flare stack, vent knockout drum, flare network and subsequent catastrophic consequences, is to continuously purge a small flow rate of hydrocarbon gases. Purge gas (or purge flow) is used to prevent air impingement in the process header where there is little or no process flow going to the flare (as in the case of a dedicated intermittent or emergency shut down flare). This continuous hydrocarbon gas purge flow in the flare network helps to build up some positive backpressure at the vent knock out drum. This continuous purge gas flow maintains a small positive backpressure in the vent knockout drum and vent header and helps to keep the air out. It is important for the source of the continuous purge gas flow to be located at the farthest end of the flare or vent network from the flare stack. This way the constant positive back pressure can be felt throughout the flare network, at all the relieving sources.
In gas plants, the total flared quantity can include the total volume due to valve leaks, purge fuel gas, relieved gas, and any other flows of gas. In some current systems, the purging flow to the flare is measured by local, variable area flow meters, which are of analog type, field readable and independent from any other system. Such local meters can be checked by a field operator on a weekly basis, and with a metallic pointer, can be difficult to accurately read, short term variations in flow can be misread or missed entirely, and the total purge gas consumption can be loosely estimated at best. There can be times when the leak rate from flare control valves, relief valves that leak process gas, fuel gas, and inert gas into the flare header exceeds the desired purge rate and it is not necessary to add purge gas.
Other current systems avoid ingress into the flare stack, flare knockout drum, and flare network using liquid seal or flame arresters. However, such methods can lead to the possibility of obstruction in the flare stack.